Influence of self-fields on the filamentation instability in relativistic beam-plasma systems

Abstract

The influence of equilibrium self-fields on the filamentation instability in relativistic beam-plasma systems is investigated within the framework of a macroscopic cold-fluid description. For a low-density beam (n̂b≪n̂e) propagating through a nonrelativistic plasma background, the instability criterion for ordinary-mode propagation perpendicular to B0zêz is shown to be (assuming perturbations with ∂/∂ϑ=0=∂/∂z) (ω2pbβ2bz/ω2cb) {1+(2ω2pb/ω2cb) [β2bz(1−fM)−(1−fq)]}−1≳1, where ωpb is the beam plasma frequency, ωcb is the beam cyclotron frequency, βbzc is the axial velocity of the beam, and fM and fq are the fractional current neutralization and fractional charge neutralization, respectively, by the background plasma. The azimuthal self-magnetic field has a stabilizing influence for fM<1, whereas the radial self-electric field has a destabilizing influence for fq<1.